Removing unnecessary substrates of a semiconductor chip to thin the chip has various advantages in improving device characteristics. For example, in the case of LEDs of gallium nitride (GaN) type fabricated by using a sapphire substrate, which has insulating properties, removing the sapphire substrate greatly improves light output. That is, eliminating the interface between a GaN-based semiconductor crystal and sapphire improves light extraction efficiency, and removing sapphire, which has a low thermal conductivity, improves heat release of the chip. These contribute significantly to the improvement of light output.
However, in the case of fabricating a thinned GaN-based LED of what is called a thin-film structure, the process of removing the sapphire substrate remains unstable and may influence manufacturing efficiency.
A laser lift-off method is considered as a technique of removing the sapphire substrate. The laser lift-off method is a method that delivers laser light, which passes through the sapphire substrate and is absorbed in the GaN-based crystal, to separate the GaN crystal at the sapphire/GaN interface. In this method, since the GaN and the sapphire are separated at the same time as the GaN is separated, residual stresses at the time of crystal growth are concentrated at the boundary between a region not irradiated with the laser and the separated region. Therefore, it is a problem that causes a crack in the GaN-type crystal.
JP-A 2004-072052 (Kokai) discloses a technology that adjusts the power of delivered laser light to form an alteration layer at the sapphire/GaN interface in order to prevent crack generation in a separated nitride semiconductor film. JP-A 2006-245043 (Kokai) discloses a technology that forms an alteration section in a sapphire substrate by laser irradiation and divides a wafer along the formed alteration section into chips.